Silica-supported perchloric acid and potassium bisulfate as reusable green catalysts for nitration of aromatics under solvent-free microwave conditions

Article abstract of DOI:10.1080/00397911.2017.1387923

Silica-supported perchloric acid and bisulfate (SiO₂/HClO₄ and SiO₂/KHSO₄) have been developed as reusable green catalysts for nitration of aromatic compounds using NaNO₂ in acetonitrile medium under conventional and solvent-free microwave conditions. The reaction times under microwave irradiation are significantly shorter than conventional method even though the yields obtained in microwave-assisted reactions are comparable with those obtained under reflux conditions.

Full text of DOI:10.1080/00397911.2017.1387923

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Silica supported perchloric acid and potassium
bisulphate as reusable green catalysts for nitration
of aromatics under solvent free microwave
conditions
M. Satish Kumar, Y. Hemanth Sriram, M. Venkateswarlu, K. C. Rajanna, M.
Sai Sudhakar, Purugula Venkanna & P. K. Saiprakash
To cite this article: M. Satish Kumar, Y. Hemanth Sriram, M. Venkateswarlu, K. C. Rajanna, M.
Sai Sudhakar, Purugula Venkanna & P. K. Saiprakash (2017): Silica supported perchloric acid
and potassium bisulphate as reusable green catalysts for nitration of aromatics under solvent free
microwave conditions, Synthetic Communications, DOI: 10.1080/00397911.2017.1387923
To link to this article: http://dx.doi.org/10.1080/00397911.2017.1387923
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Date: 30 October 2017, At: 01:02

Silica supported perchloric acid and potassium bisulphate as
reusable green catalysts for nitration of aromatics under
solvent free microwave conditions
M. Satish Kumar
Department of Chemistry, Osmania University, Hyderabad, TS, India
Y. Hemanth Sriram
Department of Chemistry, Osmania University, Hyderabad, TS, India
M. Venkateswarlu
Department of Chemistry, Osmania University, Hyderabad, TS, India
K. C. Rajanna*
Department of Chemistry, Osmania University, Hyderabad, TS, India
M. Sai Sudhakar
Department of Chemistry, BITS Pilani, Hyderabad, TS, India
Purugula Venkanna
Department of Chemistry, Osmania University, Hyderabad, TS, India
P. K. Saiprakash
Department of Chemistry, Osmania University, Hyderabad, TS, India
1

*
Address correspondence to Prof. K. C. Rajanna, Department of Chemistry, Osmania University
Hyderabad 500007, TS, India. E-mail: kcrajannaou@yahoo.com
*Supplemental data (full experimental details, and spectroscopic data) are attached a separate
file website
*
ABSTRACT
Silica supported perchloric acid and bisulfate (SiO2/HClO4 and SiO2/KHSO4) have been
developed as reusable green catalysts for nitration of aromatic compounds using NaNO2in
acetonitrile medium under conventional and solvent free microwave conditions. The reaction
times under microwave irradiation are significantly shorter than conventional method even
though the yields obtained in microwave assisted reactions are comparable with those obtained
under reflux conditions.
GRAPHICAL ABSTRACT
KEYWORDS: aromatic compounds, microwave irradiation, NANO
2
, nitration, rate
acceleration, SIO /HCLO , SIO /KHSO
2
4
2
4
Introduction
2

Nitro aromatic compounds are important precursors for various industrial products such
as pharmaceuticals, plastics and dyes. In spite of voluminous literature is reported for alternative
protocols, many of the catalysts used could not be recycled. Therefore, Nitro aromatic
compounds are yet, mainly produced by using mixed acid (nitric acid and sulfuric acid) for
nitration.[1–3] However, this classical nitration protocol is not only hazardous, but also causes
environmental pollution when the remaining (unused) nitration mixture is sent to the industry or
laboratory drains. In order to eliminate and/reduce such environmental issues, there has been an
upsurge in the development of environmentally safe chemistry protocols following Paul Anastas
[
4]
[5]
priciples. Rajesh Vekariya and Hitesh Patel summarized recent advances in the synthesis of
nitrophenols following green chemistry approches.
On the other hand, design and synthesis of reusable heterogeneous catalysts received much more
attention, because these catalysts can be removed easily by filtration and reused after completion of the
reaction. This reusability makes the whole process eco-friendly, and thus plays a prominent role in
organic chemistry due to environmental and economic considerations. In addition, heterogeneous and
reusable catalysts are superior over homogeneous catalysts. Silica supported catalyst could be placed in
the forefront because of its unique features such as high efficiency due to large surface area, high
mechanical and thermal stabilities, greater selectivity, low toxicity, reusability, and simplicity of
handling. Recently, potassium bisulphate immobilized on silica (SiO
2
-KHSO
4
) has been found to be an
efficient, recyclable heterogeneous catalyst for synthesis of propargyl indoles.[6] Literature survey reveals
that NaHSO
4
immobilized on silica (SiO
2
-NaHSO
4
) could also employed as efficient heterogeneous
catalyst for various organic transformations.[9–16] On the other hand, silica supported perchloric acid
(
SiO
2
4
/HClO ) has also been accomplished as an eco-friendly heterogeneous catalyst for various types of
organic reactions.[17–24] Encouraged by these features, we wish to report clean and environment friendly
protocols for nitration of aromatic compounds using NaNO
3
2
in presence of reusable heterogeneous green

catalysts SiO
2
-HClO
4
and SiO
2
-KHSO
4
under reflux conditions. And we have also explored the same
reactions under Microwave condition. Nevertheless, Heravi and other co-workers[25,26] recently reported
the nitration of aromatic compounds using NaNO
immobilized conditions.
2
in presence of KHSO in acetonitrile medium under un-
4
Results and Discussion
Initially, we have prepared Si-adsorbed catalyst (SiO
2
-HClO
4
and SiO
2
-KHSO ) and
4
characterized by SEM and XRD data. As shown in Figures 1 and 2. These observations are
largely similar to those reported by Zeba Siddiqui,[27] confirming the heterogeneous nature of
catalysts which comprise adsorbed KHSO
4
and HClO on silica. We have optimized the quantity
4
of the prepared catalyst required for nitration reaction using phenol as a model substrate, and
varied amounts of catalysts as shown in Table 1. Accordingly we used the optimized amount of
catalyst in the conventional and microwave assisted nitration reactions. It is also important to
note that no nitro product was observed in the absence of the catalyst.
The data compiled in Table 1 revealed that optimum quantity of SiO
nitration reaction is 0.2gms where as SiO -KHSO catalyst is 0.25gms. Further increase of
catalyst amount did not appreciably affect the yield of the product. The conversion yields of the
nitration of phenol presented in Table 1 also indicate that best results are obtained with SiO
2
-HClO catalyst for
4
2
4
2
-
HClO
4
over SiO
2
-KHSO
4
catalyst because HClO
4
is stronger acid than KHSO
4
(Perchloric acid
is a superacid with very low pKa, and HSO -
with pKa = 1.92; or K
= 1.2 x 10‒2).[28] Thus
4
a
HClO provides strong acidity with minimal interference because perchlorate is weakly
4
nucleophile, whereas KHSO
4
creates an acidic environment due to the complete protolysis of the
ion HSO
4
-. Scheme 2 shows a general conversion of aromatic compound to nitroarene.
4

To examine the reusability of the catalysts (SiO
2
-HClO
4
and SiO
2
-KHSO ) after
4
completion of the reaction, the catalysts were separated from the reaction mixture by simple
filtration, treated with ethyl acetate, and dried. The catalyst thus obtained was reused under
otherwise similar conditions (same set of experimental conditions used in initially) and used
again to corroborate the results obtained earlier. The recycled catalysts were found to be active
up to four or five cycles, with a slight reduction of catalytic activity, as shown in Figure 3.
Nitration of aromatic compounds proceeds smoothly with NaNO
2
in presence of SiO -
2
HClO
4
and SiO -KHSO under reflux conditions in acetonitrile medium. Data presented in
2
4
Tables 2 and 3 clearly demonstrate that silica supported Perchloric acid is efficient over silica
supported bisulfate in terms of reaction times and yields obtained. The reaction with phenol was
completed in 1.5hours in presence of SiO
2
-HClO
4
and where as 2.5hours in case of SiO
2
-
KHSO . In both the cases 2-nitro phenol and 4-nitro phenols were obtained as products.
4
Similarly, other aromatic compounds (entries 2-12) provided the corresponding nitro products in
good to excellent yields under similar reaction conditions. The reaction times for all the studied
reactions under conventional reflux condition are in the range of 1.5hours to 3.5hours in presence
of SiO
2
-HClO
4
, where as in the case of SiO
2
-KHSO is 2.5 to 5hours . The products obtained
4
were identified by spectroscopic methods and compared with literature reports.[29] In order to
decrease the reaction times and avoid the use of solvent, reactions were conducted under
microwave (MW) irradiation. Reaction times reduced from 1.5-5hours (under reflux) to 1-4min
in MW assisted reactions. Rate enhancements under microwave could be attributed due to bulk
activation phenomenon,[29] which is achieved due to rapid heating and pressure effects. On the
basis of observed results, a plausible mechanism for SiO
2
-HClO
4
and SiO
2
-KHSO catalyzed
4
nitration could be proposed as shown in Scheme 2. The reactions in presence of silica supported
5

catalysts with NaNO
acid thus formed might convert to nitric acid under reflux temperature. Ultimately, NO
electrophile is generated due to the reaction between HNO and silica catalysts, which then
2
gives HNO
2
in the first step, due to acidic nature of silica catalysts. Nitrous
2
+
3
interact with aromatic compound to afford nitro compound, and catalyst is regenerated as shown
in mechanistic pathway of Scheme 3.
Conclusions
In summary, we have demonstrated that nitration of aromatic compounds using NaNO
2
in
presence of SiO /HClO and SiO /KHSO (Silica supported) catalysts proceeds smoothly under
2
4
2
4
reflux and solvent free microwave conditions, with high yields, short reaction times, The chosen
catalysts could be recycled easily, showing that the developed protocols could also be included
as good examples of green chemistry methods and advantageous over several protocols.
Experimental Details
Preparation of SiO
2
/KHSO
4
and SiO
2
/HClO catalysts
4
Silica gel (4g, 100–200 mesh) was added to a solution of KHSO
water (25ml ), and stirred for about 30min at room temperature, for the adsorption of KHSO
onto the surface of the silica gel. Water was removed in vacuum to give a powder which was
dried in an oven at 120°C for 2-3hrs to afford a SiO /KHSO catalyst. Procedure for the
preparation of SiO /HClO catalyst is same as mentioned in the previous section for the
preparation of SiO /KHSO catalyst.
4
(20 mmol) in distilled
4
2
4
2
4
2
4
SEM and XRD analysis of SiO2/KHSO4 and SiO2/HClO4 catalysts
6

Scanning electron microscopic pictures of SiO
2
/KHSO
4
and SiO
2
/HClO catalysts
4
(Figures 1 and 2) under different magnifications in the range of 50 to 200µm revealed non-
uniform morphologies with polynomial cubic crystals and flakes embedded with grain and spike
like species. XRD analysis of the samples (Figure 1d) and Figure 2d) probably indicate
amorphous nature of the catalysts. Broad peaks observed around 23 and 25 respectively (on 2θ
scale) could be attributed to silica. These observations are largely similar to those reported by
Zeba Siddiqui,[27] confirming the heterogeneous nature of catalysts which comprise adsorbed
KHSO
4
and HClO on silica.
4
Nitration of aromatics using SiO2/KHSO4 under conventional conditions
The optimum amount of the catalyst (SiO
2
/KHSO
4
or SiO
2
/HClO
4
) was added to the
mixture of aromatic compound (10 mmol), NaNO
2
(10 mmol) and acetonitrile, in a previously
cleaned round bottom flask and stirred under reflux till the reaction is completed. Progress of the
reaction was monitored by TLC. After the completion of reaction, the catalyst was separated by
filtration, and the reaction mixture is treated with NaHCO
3
solution, followed by the addition of
SO , purified with column
ethyl acetate. Organic layer was then separated, dried over Na
2
4
chromatography and evaporated under vacuum to get final product. Products thus obtained were
characterized spectroscopically and obtained data were confirmed with earlier reports.[30]
Nitration of phenol in the present study afforded 2-NO
2
phenol as major product and 4-NO
2
phenol as minor component respectively.
1
2
-NO
2
Phenol: (yellow crystalline solid) HNMR (300MHz, CDCl
3
): δ 9.52 (s, 1H, OH),
8
.15 (dd, 1H, J = 8.5Hz, J = 8Hz ), 8.12 (d, 1H, J = 8Hz ) 7.55 (dd, 1H, J = 8Hz J = 7.5Hz ) 6.95
7

1
3
(
d, 1H, J = 8.5Hz ); C NMR (75MHz, CDCl ): δ 155.2, 137.6, 133.8, 125.1, 120.2, 119.9; mp =
3
o
4
3-48 C; HRMS (EI) Calcd. M, 139.1088, Found: 139.0272
1
4
-NO
2
Phenol: (yellow powder), HNMR (300MHz, CDCl
3
): δ 9.95 (s, 1H, OH), 6.95
): δ 161.7, 141.3, 126.3,
13
(
d, 2H, J = 8Hz ), 8.15 (d, 2H, J = 8Hz ); C NMR (75MHz, CDCl
3
o
1
15.8; mp = 111-113 C; HRMS (EI) Calcd. M, 139.1086, Found: 139.0274
Microwave assisted nitration of aromatics under solvent free conditions
For the purpose of microwave assisted nitration of aromatics Aromatic compound (10
mmol), NaNO
were taken in a previously cleaned beaker, mixed thoroughly and placed in micro-wave oven
CEM-908010, bench mate model, 300W laboratory microwave reactor), till the reaction is
2
(10 mmol) and optimum quantity of the catalyst (SiO
2
/KHSO
4
or SiO
2
/HClO )
4
(
completed. Progress of the reaction was monitored by TLC. After the completion, the reaction
mixture is treated with ethyl acetate, catalyst is separated by filtration. The filtrate and is washed
with NaHCO
3
solution. Organic layer was then separated, dried over Na
2
SO , purified with
4
column chromatography and evaporated under vacuum to get final product as mentioned above.
Acknowledgements
The authors are grateful to the Head (Chemistry Department), Osmania University
Hyderabad for constant support and encouragement.
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(
1
0

Table 1. Catalyst optimization for nitration of Phenol.
Entry
Phenol
Amount of catalyst (gms)
(mmol)
SiO2-HClO4
Yield (%)
SiO2-KHSO4
Yield (%)
1
2
3
4
5
6
10
10
10
10
10
10
0.1
0.15
0.20
0.25
0.3
50
65
91
92
92
–
0.1
0.15
0.20
0.25
0.3
48
58
67
76
76
77
–
0.4
1
1

Table 2. Nitration of aromatic compounds in presence of SiO2/HClO4 catalyst.
S. No
Substrate
Product
Conventional
Microwave
R. T Yield
R. T
Yield
(%)
(
hrs)
1.5
3
(min)
(%)
78:20
90
1
2
3
4
5
6
7
8
9
Phenol
β- Napthol
Anisole
2- NO2:4- NO2 Phenol
1-NO2-2-naphthol
75:16
82
1
2
2
1
2
2
2
1
2
2
2
2
4- NO2:2- NO2 Anisole
4- NO2:2- NO2 Toulene
4-Me-2-NO2 Phenol
2-Me-4-NO2 Phenol
4- NO2:2- NO2 Cl benzene
3-NO2 Indole
2
74:15
70:20
82
85:12
82:15
94
Toulene
2
p-Cresol
1.5
2.5
3
o-Cresol
76
85
Chloro benzene
Indole
65:18
80
72:18
92
2.5
3
Napthalene
Benzoic acid
Thiophene
Furan
2-NO2:1-NO2Napthalene
3- NO2 Benzoic acid
2-NO2 Thiophene
64:15
72
72:16
80
1
1
1
0
1
2
3.5
3
74
85
2- NO2 furan
2
75
82
1
2

Table 3. Nitration of aromatic compounds in presence of SiO2/KHSO4 catalyst.
S. No
Substrate
Product
Conventional
Microwave
R. T Yield (%) R. T (min) Yield (%)
(
hrs)
2.5
5
1
2
3
4
5
6
7
Phenol
β- Napthol
Anisole
2- NO2:4- NO2 Phenol
1-NO2-2-naphthol
64:12
62
2
4
4
2
3
3
3
68:15
70
4- NO2:2- NO2 Anisole
4- NO2:2- NO2 Toulene
4-Me-2-NO2 Phenol
2-Me-4-NO2 Phenol
3
65:15
58:20
75
70:16
60:22
82
Toulene
3
p-Cresol
3
o-Cresol
3.5
4
70
74
Chloro benzene
4- NO2:2- NO2 Cl
benzene
54:15
62:18
8
9
Indole
3-NO2 Indole
2.5
4
78
2
3
85
Napthalene
2-NO2:1-
50:16
62:15
NO2Napthalene
1
1
1
0
1
2
Benzoic acid
Thiophene
Furan
3- NO2 Benzoic acid
2-NO2 Thiophene
2- NO2 furan
4
3.5
3
68
65
70
4
4
3
75
70
75
1
3

Figure 1. (a) SEM of Si-KHSO4 with 100µm magnification. (b): XRD analysis of SiO2/ KHSO4
catalyst.
1
4

Figure 2 (a): SEM of Si-HClO4 with 100µm magnification. (b): XRD analysis of Si-HClO4
catalyst.
1
5

Figure 3. Bar diagram for recyclability SiO2-HClO4 and SiO2-KHSO4 catalysts.
1
6

Scheme 1. Nitration of aromatic compounds using Si-supported HClO4 and KHSO4 as catalysts
in presence of NaNO2.
1
7

Scheme 2. Adsorption of HClO4 and KHSO4 on SiO2 to get SiO2-HClO4 and SiO2-KHSO4.
1
8

Scheme 3. Mechanism of nitration in presence of SiO2-HClO4 and SiO2-KHSO4.
1
9